Novel causative mutations in patients with Nance-Horan syndrome and altered localization of the mutant NHS-A protein isoform.
OBJECTIVE
Nance-Horan syndrome is typically characterized by severe bilateral congenital cataracts and dental abnormalities. Truncating mutations in the Nance-Horan syndrome (NHS) gene cause this X-linked genetic disorder. NHS encodes two isoforms, NHS-A and NHS-1A. The ocular lens expresses NHS-A, the epithelial and neuronal cell specific isoform. The NHS-A protein localizes in the lens epithelium at the cellular periphery. The data to date suggest a role for this isoform at cell-cell junctions in epithelial cells. This study aimed to identify the causative mutations in new patients diagnosed with Nance-Horan syndrome and to investigate the effect of mutations on subcellular localization of the NHS-A protein.


METHODS
All coding exons of NHS were screened for mutations by polymerase chain reaction (PCR) and sequencing. PCR-based mutagenesis was performed to introduce three independent mutations in the NHS-A cDNA. Expression and localization of the mutant proteins was determined in mammalian epithelial cells.


RESULTS
Truncating mutations were found in 6 out of 10 unrelated patients from four countries. Each of four patients carried a novel mutation (R248X, P264fs, K1198fs, and I1302fs), and each of the two other patients carried two previously reported mutations (R373X and R879X). No mutation was found in the gene in four patients. Two disease-causing mutations (R134fs and R901X) and an artificial mutation (T1357fs) resulted in premature truncation of the NHS-A protein. All three mutant proteins failed to localize to the cellular periphery in epithelial cells and instead were found in the cytoplasm.


CONCLUSIONS
This study brings the total number of mutations identified in NHS to 18. The mislocalization of the mutant NHS-A protein, revealed by mutation analysis, is expected to adversely affect cell-cell junctions in epithelial cells such as the lens epithelium, which may explain cataractogenesis in Nance-Horan syndrome patients. Mutation analysis also shed light on the significance of NHS-A regions for its localization and, hence, its function at epithelial cell junctions.
Sharma
Shiwani
S
Department of Ophthalmology, Flinders University, Bedford Park, South Australia, Australia. shiwani.sharma@flinders.edu.au
Burdon
Kathryn P
KP
Dave
Alpana
A
Jamieson
Robyn V
RV
Yaron
Yuval
Y
Billson
Frank
F
Van Maldergem
Lionel
L
Lorenz
Birgit
B
Gécz
Jozef
J
Craig
Jamie E
JE
D000015:Abnormalities, Multiple / Q000235:genetics*; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D004252:DNA Mutational Analysis; D004285:Dogs; D006801:Humans; D008297:Male; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D016296:Mutagenesis; D050505:Mutant Proteins / Q000378:metabolism*; D009154:Mutation / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D013577:Syndrome
D008565:Membrane Proteins; D050505:Mutant Proteins; C488621:NHS protein, human; D009687:Nuclear Proteins; D020033:Protein Isoforms